1. Field of the Invention
The present invention relates to a method and an apparatus for determining the layer thickness and the refractive index of a sample.
2. Description of Related Art
Measurements of layer thickness and refractive index belong to the most important tools during quality control in the semiconductor production. In these days, such measurements are not only carried out for quality assurance after the manufacturing process steps, but also through real-time measurements during coating processes.
In this respect, different spectroscopic-optical real-time methods are known in the prior art. Light is suitably irradiated onto the sample (consisting of a thin layer on a substrate) to be investigated and then measured either in reflection or in transmission in order to achieve a contactless determination of the layer thickness. Typical measurement methods using perpendicular incidence of light are transmission-spectroscopy and reflectance-spectroscopy. Furthermore, U.S. Pat. No. 5,999,267 discloses an apparatus comprising two light sources for irradiating a film at a normal angle and at an oblique angle of incidence, respectively, wherein the calculation of the optical constants and thickness of the film is based on the power spectral density spectra of the two reflection spectra. Typical measurement methods using oblique incidence of light are ellipsometry and polarisation dependent photometry. Examples of methods using two light sources irradiating at different oblique incidence angles are discussed in “Optical constants of a sodium alginate polymer in the UV-vis range” by Ó. Esteban et al., Optical Materials 31 (2009), 696-699, and in “Optical characterization of β-FeSi2 thin films prepared on fused quartz by femto second laser ablation” by Youhua Zhou et al., Physica B 399 (2007) 33-37.
The change of the light intensity or the shift of the light phase caused by the layer structure is then measured. These changes can be described by physical laws so that the change of the light intensity/the shift of the light phase are a function of the layer parameters, i.e. a function of the layer thickness and the optical material properties of the sample. Thus, the layer parameters may be determined using said functional dependence. Due to the fact that there is a strongly non-linear dependence between the change of the light intensity/shift of the light phase and the layer parameters, the mathematical determination is not performed by analytical calculations but by numerical fitting algorithms as for example Marquardt-Levenberg and Simplex of Nelder & Meat.
However, the methods disclosed in the prior art can only be reasonably used for samples that comprise a relatively low surface and interface scattering. Due to the fact that in solar cell applications (using wet chemical etching) silicon wafers are intentionally manufactured having a textured surface (on which a thin transparent film is applied) with relatively high scattering characteristics in order to increase the solar cell efficiency, the methods disclosed in the prior art cannot be appropriately used for an fast and exact determination of the layer thickness and the refractive index of such highly scattering samples which are e.g. shown in FIG. 1.
In particular, it is disadvantageous in the prior art that known methods such as ellipsometry come along with high adjustment requirements or result in a low measurement accuracy such as the camera based reflection colour recognition. Moreover, the reflection colour recognition only provides the product of layer thickness d and refractive index n but not the required single values.
It is therefore an object of the present invention to provide a method for determining the layer thickness of a sample (substrate having a thin layer thereon, wherein the thickness of the thin layer is to be determined) having high light scattering characteristics that allow a fast (real-time process) and cost-effective measurement having a high accuracy. In particular, the method and the apparatus of the present invention shall be applicable in manufacturing processes for wafer based solar cells in which the solar cell structures (samples) can be investigated for no more than a few tens of a milliseconds during process control.